The problem of separating out or disambiguating interfering digital transmissions from different users has been resolved in the past by utilizing multi-user detectors as described in Verdu, “Multiuser Detector”, Cambridge University Press, 1998 and by Wang and Poor, “Iterative Turbo Soft Interference Cancellation and Decoding for Coded CDMA, IEEE Transactions on Communications,” July, 1999, as well as a patent application Ser. No. 09/923,709, filed by Rachel Learned et al on Aug. 7, 2001, entitled Method For Overusing Frequencies To Permit Simultaneous Transmission Of Signals From Two Or More Users On The Same Frequency and Time Slot, and incorporated herein by reference.
For full multi-user detection systems which examine the entire universe of possibilities for each bit from each user, the processing time is inordinately long. The processing time is, of course, exponentially proportional to the number of interfering signals that exist. In general, processing time has been reduced by several suboptimal methods through the examination of less than the entire number of possibilities for a given bit from a given user. A Turbo MUD is one such example.
More particularly, for terrestrial-based CDMA wireless communication systems, if it were possible to be able to accommodate a larger number of users on the same channel, meaning transmitting at the same frequency at the same time, then network utilization could be increased dramatically. CDMA systems exist that can accommodate a given number of simultaneous users and in general, use matched filtering techniques for separating out the signals. When, however, these systems have more users than they were designed for, or practical limitations result in a fewer number of users than can actually be accommodated, then a so-called over-saturated condition occurs.
One way of addressing the over-saturation issue is to utilize multi-user detectors to be able to separate out the interfering signals. One of the problems with the utilization of multi-user detectors, MUDs, in over-saturated environments is the processing time necessary to be able to separate out the interfering signals. In some over-saturated cases, only full MUD processing is available to be able to separate out the signals.
It will be appreciated that iteratively processing MUDs, called Turbo MUDs, are generally designed as a low-complexity way of being able to duplicate the functions of a full MUD. The low-complexity MUDs use considerably less processing time than full MUDs, making possible real-time recovery of the interfering signals. Note that Turbo MUDs in general utilize the information in forward error correction codes to be able to accomplish their task.
The result is that while Turbo MUD techniques have considerably improved processing times with respect to separating out the interfering signals, when either over-saturation occurs or becomes severe; or in fact, when processing times are to be reduced regardless, it is important to be able to quickly arrive at a value for a given bit for a given user, meaning that the error rate drops below an acceptable level for the particular communication scheme involved.
For an entire class of Turbo MUDs, the MUD processes the incoming signals on a time instant by time instant basis to provide for each user a series of best guesses as to what the individual bit should be for a given time. These best guesses, commonly known as soft outputs, are utilized in an error correction device, usually a single-user detector such as a BCJR Decoder, the purpose of which is to provide error correction for the bits for each user by comparing the present bit to a prior bit and/or a subsequent bit.
The result of such an error correction is a matrix of bit estimates which ordinarily are used by the MUD in the iterative process to provide better estimates of the particular bit. The number of iterations necessary to reduce the error rate to a predetermined acceptable level is a function of a number of factors, most notably the number of simultaneous users.
In the past, such processing has taken a long period of time, so long that the result is considered a non-real time response.
For instance, the conventional, minimum mean squared error, MMSE, multi-user detector which is described by Wang and Poor mentioned above, is relatively slow. It involves an iterative process for separating interfering signals in which the output of an error corrector is fed back to a multi-user detector for the purpose of obtaining better guesses as to the individual bits in the incoming data stream on the subsequent iterations.